This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to application Ser. No. 0024702.3 filed in the United Kingdom on Oct. 9, 2000; the entire content of which is hereby incorporated by reference.
The invention relates to the measurement of heat flux in heated chambers, for example, in ovens of the kind that are suitable for use in continuous processes in which material to be heated (which may be in the form of discrete articles) is transported through the oven and is heated progressively during its passage through the oven. Such ovens are known as tunnel ovens because they are elongate and have at one end an entrance through which the material is introduced into the oven and, at the other end, an exit through which the material is withdrawn. Tunnel ovens are used for a variety of purposes, for example, to dry material or to effect the baking of food products.
The heat flux to be measured is that incident on a surface of the material to be heated. In general, the heat flux will have radiative and convective components. The material will normally be supported from below on the upper run of an endless band conveyor and, where the band is imperforate, the only exposed surface of the material will be its upper surface. Where the band is a mesh, the lower surface of the material will be partly exposed to the heat flux, but it is the heat flux incident on the upper surface of the material that is here of prime concern.
It is often important to measure separately the radiative and convective components of the heat flux, and that can be done by comparing measurements made using a radiation-absorbing sensor with measurements made using a reflecting sensor. In each case, the heat flow can be determined by measuring, together with certain other quantities, the temperature difference across a thermally insulating layer located between the exposed surface of the sensor and a heat sink. Essentially, the radiation-absorbing sensor responds to the total heat flux whereas the reflecting sensor responds only to the convective component of the heat flux.
Of course, neither sensor will behave either as a black body or as a perfect reflector; each sensor will both absorb and reflect radiation incident on it. It is strictly necessary only that the two sensors should have different absorptivities, but the more nearly the radiation-absorbing sensor behaves as a black body and the more nearly the reflecting sensor behaves as a perfect reflector the better the apparatus will perform. Throughout the specification, references to a surface or a sensor being radiation-absorbing or radiation-reflecting are to be understood as taking account of those facts.
An apparatus have two such sensors and arranged to operate in that way is described in UK Patent Specification No 2 183 346B. Typically, the heat flux will vary significantly along the length of a tunnel oven, with considerable variations occurring over relatively small distances. That will be especially marked when, for example, the heat flux is primarily radiative and derives from burners or other heating elements extending across the width of the oven at intervals along its length. Thus, the axial profile of the heat flux will show pronounced peaks and troughs, and the apparatus of the invention is intended to enable the precise form of that profile to be ascertained.
Where there is high spatial frequency of the fluctuations of the heat flux along the length of the oven, it seems clear that the two sensors must pass through the oven side-by-side. In a well designed oven, variations in the heat flux across the width of the oven will be small, but they will not usually be entirely negligible. Therefore, in order to minimise the effect of those variations across the width of the oven, the two sensors must be situated close together.
In International Specification no. WO98/09143, the relatively large variations in the heat flux along the length of the oven and the relatively smaller variations across the width of the oven are taken account of by conveying through the oven, one behind the other, a radiation-absorbing sensor and a radiation-reflecting sensor, and time-adjusting the readings from the sensors according to their speed of travel such that readings corresponding to the same position along the length of the oven are compared.
It has now been found that, by adoption of certain configurations of sensors, it is possible to compensate for the variations in heat flux along and across a heated chamber, especially a tunnel oven, without the need to provide for time-adjustment of readings.
The invention provides a heat flux measuring device for transporting through a heated chamber, the device having an array of sensors, each sensor comprising first and second surfaces bounding a region, a thermally insulating layer substantially occupying said region, and means for providing a signal which is a measure of the temperature difference across the layer, said first surface of each sensor being in thermal contact with a heat sink and said second surface of each sensor being exposed, a plurality of the said exposed surfaces of the array being reflective to radiation and a further plurality of the said exposed surfaces of the array being absorbent to radiation, the sensors of the array being so arranged that the area of radiation-absorbing exposed sensor surface and the area of radiation-reflecting exposed sensor surface are each equally distributed about a mid-line of the array extending along the direction in which in use the device will be transported and about a mid-line of the array extending along the direction transverse to the direction in which in use the device will be transported.
The device of the invention can be of relatively simple construction, but nevertheless make allowance for variations in heat flux within the chamber and thus permit relatively accurate measurements of the heat flux to be made in a relatively straightforward manner.
Each reflecting exposed surface may be a surface of the insulating layer of the respective sensor. Each reflecting exposed surface may instead comprise a layer of heat-conductive material having a reflecting surface that is in thermal contact with the insulating layer of the respective sensor.
Each absorbing exposed sensor surface may be a surface of the insulating layer of the respective sensor. Each absorbing exposed sensor surface may instead comprise a layer of heat-conductive material having a radiation-absorbing surface that is in thermal contact with the insulating layer of the respective sensor. Advantageously, the array is so arranged that the reflecting and absorbing surfaces form portions of a substantially continuous surface. Thus, in a preferred form of the device, the exposed sensor surfaces may each be a portion of a continuous layer, for example, of a flexible sheet of material which is arranged to be reflecting in selected regions and arranged to be radiation-absorbing in other regions. For example, the sensor surfaces may each be a region of sheet of a metal foil, for example an aluminium foil, of which selected regions have been treated, for example, by blackening with a matt black paint, to render them absorbing to radiation.
It is preferred for the radiation-reflecting and radiation-absorbing exposed sensor surfaces to be spaced from one another. In that case, the separation between the radiation-reflecting surfaces and the radiation-absorbing surfaces is advantageously at least 5 mm.
The device may be arranged to generate a cumulative signal representative of the sum of the individual sensor signals of those sensors having a reflecting surface. In general, that will be achieved by connecting in series those sensors having a reflecting surface. Similarly, the device may be arranged to generate a cumulative signal representative of the sum of the individual sensor signals of those sensors having an absorbing surface, and that will in general be achieved by connecting in series those sensors having a radiation-absorbing surface. Such arrangements of the device are preferred, but it is also possible to arrange for the individual sensor signals to be summed or averaged in data processing means associated with the device. The said associated data processing means may be arranged to travel with the device, in use, or be situated outside the heated chamber.
Advantageously, there is provided means for comparison of a cumulative signal representative of the signals of those individual sensors having a reflecting surface with a cumulative signal representative of the signals of those individual sensors having an absorbing surface. Such a comparison of the cumulative signal for the reflecting sensors with the cumulative signal for the absorbing sensors can provide an indication of radiant heat received by the sensors. As mentioned previously, the radiation-absorbing sensors will not normally be perfectly absorbing and the radiation-reflecting sensors will not normally be perfectly reflecting. For practical purposes, the comparison result so obtained may be adequate. If desired, however, the actual value of the radiation heat flux may be calculated using the two cumulative values, provided that certain other parameters are known or can be measured (see equations (1) and (2) below).
Each of the said exposed sensor surfaces of the array is preferably of substantially equal area. Preferably, the exposed surfaces of the array together have an overall substantially rectangular, or substantially square, configuration. Preferably, the individual reflecting surfaces and the individual absorbing surfaces are each substantially rectangular or substantially square.
Preferably, the array comprises three or more radiation-reflecting sensor surfaces and three or more radiation-absorbing sensor surfaces. Preferably, the array comprises at least two parallel rows of alternate reflecting sensor surfaces and absorbing sensor surfaces extending in the direction in which in use the device travels. Preferably, the array comprises at least two parallel lines of alternate reflecting sensor surfaces and absorbing sensor surfaces extending in the transverse direction. It will be appreciated that the number of rows and the number of transverse lines of alternate reflecting and absorbing surfaces will preferably be an even number where the reflecting and absorbing exposed sensor surfaces are substantially of equal size.
Whilst it is preferable for the overall configuration of the exposed surfaces of the array to be substantially rectangular or substantially square, in each case having rectangular or square individual exposed sensor surfaces, other configurations may be suitable. For example, the overall configuration may be round (for example, circular), and the individual exposed sensor surfaces may each be located within a respective sector of the round configuration, with alternating radiation-absorbing and radiation-reflecting sectors, or even with radiation-absorbing and radiation-reflecting sensor surfaces alternating in both circumferential and radial directions.
Advantageously, in each sensor a thermopile constitutes the layer of a thermally insulating material and the means for providing a signal representing the temperature difference across the layer. A thermopile comprises a thin layer of an insulating material, for example, a piece of thin film of a plastics material, in which are embedded thermocouples connected in series and so arranged that the cold junction of each thermocouple is located close to one surface of the layer and the hot junction of each thermocouple is located close to the other surface of the layer. The use of a plurality of thermocouples connected in series both gives a spread of readings over the area where the thermocouple junctions are situated and, more importantly, gives a larger signal for a given temperature difference. Further because of the construction of a thermopile, it is possible to arrange that it has a small thermal capacity and hence a short response time.
It will be appreciated that the expression xe2x80x9cthermally insulating layerxe2x80x9d includes not only arrangements having a monolithic layer of thermally insulating material, but also arrangements in which the layer is made up of two or more sub-layers of thermally insulating material.
Where, as in the case of thermopiles, the temperature difference is determined using electrical means, it may be necessary to provide electrical insulation between the electrical means and certain adjacent parts of the device. For example, a thin electrically insulating layer may be provided to insulate the thermopile electrically from any additional layer of heat-conductive material that forms the absorbing surfaces and/or the reflecting surfaces of the sensor of which the thermopile forms a part.
It is desirable that the area of the exposed surfaces of the sensors should be large because that permits the use of thermopiles with a large number of junctions and so provides a larger signal representing the temperature difference across the layer of a thermally insulating material. On the other hand, it has been found that increasing the linear dimension of the exposed surfaces of the sensors in the direction of travel of the apparatus through the oven impairs the spatial resolution of the apparatus for the variations in heat flux in that direction.
Preferably, the device comprises means for ascertaining the temperature of the radiation-absorbing sensor surfaces and/or means for ascertaining the temperature of the radiation-reflecting sensor surfaces. In each case, those means advantageously comprise a thermocouple junction located at at least one exposed absorbing surface and a thermocouple junction located at at least one exposed reflecting surface.
Preferably, the device comprises means for ascertaining the gas temperature in the vicinity of the exposed surfaces of the sensors. Although, in principle, measurement of gas temperature in the vicinity of both the reflecting surfaces and the absorbing surfaces would give improved accuracy, it has been found that the variation in gas temperature is not usually sufficient within the vicinity of the device for it to be necessary to take that into account. The means for ascertaining the gas temperature may, for example, be a flexible air thermocouple.
Advantageously, the heat sink is a unitary body of heat conductive material. The heat sink may comprise a metal block. If desired, the sensors may be mounted on portions of the metal block that are raised, for example by at least 5 mm, above the upper surface of the remainder of the block. The sensors are in thermal contact with the heat sink. It is not necessary for the sensors to be in direct contact with the heat sink, provided that thermal conduction between the sensor and heat sink is efficient. In many cases, for example where as described above the sensor includes a thermopile, the sensor should be electrically insulated from the heat sink, for example, by a thin layer of electrically insulating material which does not inhibit to a material extent thermal conduction between the sensor and the heat sink.
In practice, the device may be mounted in a casing which has a recess for receiving the device such that the exposed sensor surfaces are substantially coplanar with the unrecessed region of the casing surrounding the recess. The apparatus, including the casing and the device, is advantageously provided with a removable extension arranged to provide a horizontal surface substantially coplanar with the upper surface of the casing and with the upper surface of the device located therein, and extending away from one end of the casing, the extension extending beyond the lateral extremities of the sensor device. The provision of such a plate eliminates or reduces the effect on the measurements that might otherwise result from the disturbance of the gas flow within the oven caused by the presence of the apparatus. The plate may be detachable, for example if the apparatus is to be used to measure the heat flux close to a side of the oven.
The invention further provides a method of measuring heat flux in a heated chamber, comprising conveying through at least a portion of the chamber a heat flux measuring device having an array of sensors, a plurality of the sensors each having an exposed surface which is reflecting to radiation and a further plurality of the sensors each having an exposed surface which is absorbing to radiation and the sensors of the array being so arranged that the area of radiation-absorbing exposed sensor surface and the area of radiation-reflecting exposed sensor surface are each substantially equally distributed about a mid-line of the array extending along the direction in which the device is transported and about a mid-line of the array extending along the direction transverse to the direction in which the device is transported, monitoring the outputs of the respective sensors and deriving from the said outputs a value which is representative of the heat flux.
Advantageously, the output of those sensors of which the exposed surface is reflecting to radiation is summed. Advantageously, the output of those sensors of which the exposed surface is absorbent to radiation is summed. Preferably, the output of those sensors having a reflecting surface is summed to generate a first summation product relating to convective heat flux, and the output of those sensors having an absorbing surface is summed to generate a second summation product relating to the combined radiative and convective heat flux, and a value for the radiative component is calculated from said first and second summation products.